What would you say to a friend that was a flat earthist?
update: She has blocked me so i can't tell her off anymore for believing in the flat earth garbage.
I would be immediately judging their intelligence and mental state, but would be compelled to ask questions. "WHY do you believe that? HOW do you believe it, with all the evidence that it isn't true? Have you ever flown in an airplane?" You can literally see the curvature of the Earth...!!
Tell them to grab a pair of binoculars and go sailing. When you approach another sailboat on the horizon the first thing you can see is the top of the mast. As the boat gets closer you can see further down the mast until you finally see the hull. If the surface of the ocean was flat you’d see the whole boat emerge at once. This is why sailors were the first to realize the surface of the earth is curved.
Alternately, just continue sailing, and you’ll end up where you started.
Some people prefer not to be taken to task for things they genuinely believe -- whether it's a flat earth or a young earth or whatever. (Of course, I suppose no one prefers to be taken to task.) But I suspect there are others who have real doubts, yet for various reasons prefer not to give into them, and again do not wish to be taken to task. It's easier to just avoid the external conflict which keeps the internal conflict stirred up. Just guessing.
I have a friend who believes in a young earth. I made the decision that the friendship was more important, and neither of us brings up the subject, and it works. But sometimes, I admit... I really want to say something. I know it's difficult.
I would have first politely asked my friend, or, my fellow human who believes this planet is flat, why he/she believes this to be true. I would allow my friend to present their position and ask if I could then explain to them that gravity is a thing. I would ask if they had ever heard of Galileo, Sir Isaac Newton or of Albert Einstein. I would ask if they recall a 3rd grade lesson involving disappearing ships at the horizon of open seas and if they remembered that the lesson was called Science and the topic was to demonstrate that Earth is indeed round? If my fellow human being does not grasp this concept nor agree, I would not argue with them. You simply can not make a logical argument with this person. You can agree on one thing: (Except for 1 or 2 scientists in the early 1600s) We all believed the earth was flat.
I know one person who is the trifecta of anti-vaxxer, creationist, and flat-earther. I think she has her particular religion as part of her identity, so everything scientific or historical fact challenges those beliefs. It's a defense mechanism to question the validity of science and logic itself.
Last I heard she and her flat-earther boyfriend are actively trying to prove the earth is flat by running experiments. So maybe she's on the way out of that illogical cesspool, hopefully. But I doubt it.